Permanent magnet type three-phase stepping motor

ABSTRACT

A permanent magnet type three-phase stepping motor that is simple in construction, with reduced vibration, a reduced unbalanced force in the radial direction, and high torque that can be manufactured at a low cost. The permanent magnet type three-phase stepping motor has a stator and a rotor arranged concentrically with the stator and with an air gap therebetween, said stator having three stator poles, and stator windings of three-phase each wound around each stator pole, each of said stator poles having m pieces of stator pole tooth, said rotor having two split rotor elements, a permanent magnet held between the rotor elements and magnetized so as to form N and S poles in the axial direction thereof, and a plurality (Nr) of small rotor teeth formed at a regular pitch on the outer peripheral surface of each of said rotor elements, said two split rotor elements being circumferentially shifted from each other by a ½ pitch of the small rotor teeth, wherein a pitch of the stator pole teeth is smaller than the pitch of the small rotor teeth, m is an integer and (Nr±2 k )/3, Nr is 3n±1, n is an integer not less than 1, and k is an integer not less than 1 and not more than 4.

RELATED APPLICATION

This application is a division of application Ser. No. 10/196,982 filedJul. 17, 2002, now U.S. Pat. No. 6,903,476, which claims priority toJapanese Application No. 223010-2001 filed Jul. 24, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a permanent magnet type three-phasestepping motor and, more particularly, to a permanent magnet typethree-phase stepping motor for use in an apparatus for an officeautomation or the like.

2. Description of the Prior Art

The vibration generated in the permanent magnet type three-phasestepping motor is smaller than the vibration generated in the permanentmagnet type two-phase stepping motor. The number of transistors requiredfor the driving circuit of the three-phase stepping motor is six,whereas the number of transistors required for the driving circuit ofthe two-phase stepping motor is eight. Accordingly, the three-phasestepping motor is excellent in cost performance. Further, the precisionof the positioning of the rotor and the fluctuation of rotation at thelow speed can be enhanced if the pole pair number of the rotor isincreased. However, the vibration and the noise are generated even inthe three-phase stepping motor, because many harmonic waves aregenerated in the magnetic flux field made by the permanent magnet underthe affection of a number of pole teeth.

It is considered that the number of the stator main poles in thethree-phase stepping motor is a multiple of three, such as 3, 6, 9, 12,. . . . The motor having three stator main poles is the most simple inconstruction. Further, it is noted that a lesser number of stator mainpoles causes larger torque, because the interlinkage magnetic flux perone stator main pole becomes large. Japanese Patent No. 3140814 and thecorresponding U.S. Pat. No. 5,289,064 are known as the prior art.

If such conventional stepping motor having three stator main poles isdriven by the two-phase exciting manner, an unbalanced electromagneticforce is generated in the radial direction, and harmonic waves aregenerated due to the large number of small rotor teeth, so that thecogging torque is increased. Further, the center of the air gap betweenthe rotor and the stator is deviated due to the unbalancedelectromagnetic force in the radial direction, so that the vibration andthe cogging torque are also increased.

In the stepping motor having twelve stator main poles, punched siliconsteel plates can be piled while changing in angular position by 90degrees so as to form the stator core, so that the differences inproperty of the silicon steel plates due to the rolling directionthereof are cancelled with one another so as to improve the magneticbalance, that the harmonic waves can be reduced because the density ofthe magnetic flux in the air gap becomes substantially in the form ofsine wave, that the fluctuation in thickness of the stator core becomessmall, and that the air gap becomes uniform. However, in case that thenumber of the stator main poles is six or nine, the punched siliconsteel plates cannot be piled while changing in angular position by 90degrees, so that the cogging torque becomes larger than that in casethat the number of the stator main poles is twelve, because anoscillation torque is added to the torque generated according to theFleming's Left-Hand Rule, that is, IBL rule ( where I is current, B ismagnetic flux and L is thickness of piled plates ), when the windingsare excited by the lower order, such as third or fourth harmonic wave.

Further, in the stepping motor of the conventional construction, if anumber Nr of the small rotor teeth becomes large a tooth phase (mechanical angle ) between the teeth of N pole and S pole of the rotoris 180/Nr and becomes small. Accordingly, the error in the electricalangle becomes large even if the error in the mechanical angle is small.Further more, in case that the number of the stator main poles is threeor six, the punched silicon steel plates cannot be piled while changingin angular position by 90 degrees, so that the harmonic wave ofrelatively low order generated due to the unbalance of the magneticresistance in the magnetic circuit or the unevenness of the air gap isnot disappeared between the phases.

Thus, the manufactured stepping motors become different from one anotherin cogging torque, vibration or noise.

SUMMARY OF THE INVENTION

In order to solve the foregoing problem in the conventional steppingmotor, it is an object of the present invention to provide a permanentmagnet type three-phase stepping motor comprising a stator and a rotorarranged concentrically with the stator and with an air gaptherebetween, said stator having three stator poles, and stator windingsof three-phase each wound around each stator pole, each of said statorpoles having m pieces of stator pole tooth, said rotor having two splitrotor elements, a permanent magnet held between the rotor elements andmagnetized so as to form N and S poles in the axial direction thereof,and a plurality (Nr) of small rotor teeth formed at a regular pitch onthe outer peripheral surface of each of said rotor elements, said twosplit rotor elements being circumferentially shifted from each other bya ½ pitch of the small rotor teeth, or said rotor being a cylindricalpermanent magnet magnetized so as to form a plurality (Nr) of N and Spole pairs alternately at a regular pitch in the circumferentialdirection thereof, wherein a pitch of the stator pole teeth is smallerthan the pitch of the small rotor teeth or rotor pole, m is an integerand (Nr±2^(k))/3, Nr is 3n±1, n is an integer not less than 1, and k isan integer not less than 1 and not more than 4.

It is another object of the present invention to provide a permanentmagnet type three-phase stepping motor comprising a stator and a rotorarranged concentrically with the stator and with an air gaptherebetween, said stator having three stator poles, and stator windingsof three-phase each wound around each stator pole, each of said statorpoles having m pieces of stator pole tooth, said rotor having two splitrotor elements, a permanent magnet held between the rotor elements andmagnetized so as to form N and S poles in the axial direction thereof,and a plurality (Nr) of small rotor teeth formed at a regular pitch onthe outer peripheral surface of each of said rotor elements, said twosplit rotor elements being circumferentially shifted from each other bya ½ pitch of the small rotor teeth, or said rotor being a cylindricalpermanent magnet magnetized so as to form a plurality (Nr) of N and Spole pairs alternately at a regular pitch in the circumferentialdirection thereof, wherein a pitch of the stator pole teeth inelectrical angle is 2π±α, α is a deviation in electrical angle betweenthe pitch of the small rotor teeth or rotor pole and the pitch of thestator pole teeth, m is an even number not less than 4, each of α₁ andα₂ is an electrical angle, α₂≦α≦α₁, and α₁ and α₂ are obtained byFormula 1 and Formula 2.

$\begin{matrix}{{\sum\limits_{k = 1}^{m/2}\;{\cos\; 2( {k - \frac{1}{2}} )\alpha_{1}}} = 0} & (1) \\{{\sum\limits_{k = 1}^{m/2}\;{\cos\; 7( {k - \frac{1}{2}} )\alpha_{2}}} = 0} & (2)\end{matrix}$

It is further object of the present invention to provide a permanentmagnet type three-phase stepping motor comprising a stator and a rotorarranged concentrically with the stator and with an air gaptherebetween, said stator having three stator poles, and stator windingsof three-phase each wound around each stator pole, each of said statorpoles having m pieces of stator pole tooth, said rotor having two splitrotor elements, a permanent magnet held between the rotor elements andmagnetized so as to form N and S poles in the axial direction thereof,and a plurality (Nr) of small rotor teeth formed at a regular pitch onthe outer peripheral surface of each of said rotor elements, said twosplit rotor elements being circumferentially shifted from each other bya ½ pitch of the small rotor teeth, or said rotor being a cylindricalpermanent magnet magnetized so as to form a plurality (Nr) of N and Spole pairs alternately at a regular pitch in the circumferentialdirection thereof, wherein a pitch of the stator pole teeth inelectrical angle is 2π±α, α is a deviation in electrical angle betweenthe pitch of the small rotor teeth or rotor pole and the pitch of thestator pole teeth, m is an odd number not less than 3, each of α₁ and α₂is an electrical angle, α₂≦α≦α₁, and α₁ and α₂ are obtained by Formula 3and Formula 4.

$\begin{matrix}{{1 + {\sum\limits_{k = 1}^{{({m - 1})}/2}{\cos\; 2k\;\alpha_{1}}}} = 0} & (3) \\{{1 + {\sum\limits_{k = 1}^{{({m - 1})}/2}{\cos\; 7k\;\alpha_{2}}}} = 0} & (4)\end{matrix}$

It is still further object of the present invention to provide apermanent magnet type three-phase stepping motor comprising a stator anda rotor arranged concentrically with the stator and with an air gaptherebetween, said stator having 3 L pieces of stator pole, and statorwindings of three-phase each wound around each stator pole, each of saidstator poles having a plurality of stator pole teeth, said rotor havingtwo split rotor elements and a permanent magnet held between the rotorelements and magnetized so as to form N and S poles in the axialdirection thereof, and a plurality (Nr) of small rotor teeth formed at aregular pitch on the outer peripheral surface of each of said rotorelements, said two split rotor elements being circumferentially shiftedfrom each other by a ½ pitch of the small rotor teeth, or said rotorbeing a cylindrical permanent magnet magnetized so as to form aplurality (Nr) of N and S pole pairs alternately at a regular pitch inthe circumferential direction thereof, wherein a condition of0.3≦{(Nr/π)(Ws+Wr)/(Ds+Dr)}≦0.5 is established, where L is an integernot less than 1, Nr is L(3n±1), n is an integer not less than 1, Ds isan inner diameter of the stator, Dr is an outer diameter of the rotor,Ws is a width of the stator pole tooth, and Wr is a width of the rotortooth or rotor pole.

It is yet further object of the present invention to provide a permanentmagnet type three-phase stepping motor comprising a stator and a rotorarranged concentrically with the stator and with an air gaptherebetween, said stator having 3K pieces of stator pole, and statorwindings of three-phase each wound around each stator pole, each of saidstator poles having a plurality of stator pole teeth, said rotor havingtwo split rotor elements and a permanent magnet held between the rotorelements and magnetized so as to form N and S poles in the axialdirection thereof, and a plurality (Nr) of small rotor teeth formed at aregular pitch on the outer peripheral surface of each of said rotorelements, said two split rotor elements being circumferentially shiftedfrom each other by a ½ pitch of the small rotor teeth, or said rotorbeing a cylindrical permanent magnet magnetized so as to form aplurality (Nr) of N and S pole pairs at a regular pitch in thecircumferential direction thereof, wherein a condition of0.3≦{(Nr/π)(Ws+Wr)/(Ds+Dr)}≦0.5 is established, where K is an evennumber not less than 2, Nr is K/2(6n±1), n is an integer not less than1, Ds is an inner diameter of the stator, Dr is an outer diameter of therotor, Ws is a width of the stator pole tooth, and Wr is a width of therotor tooth or rotor pole.

The three-phase exiting is carried out with respect to the three-phasewindings and a current passing through each of the three-phase windingsis controlled so as to eliminate an unbalanced force in the radialdirection.

The rotor is held by a bracket of non-magnetic material having a portionextending inwardly in the axial direction for supporting the innerperipheral surface of the stator poles.

These and other aspects and objects of the present invention will bebetter appreciated and understood when considered in conjunction withthe following description and the accompanying drawings. It should beunderstood, however, that the following description, while indicationpreferred embodiments of the present invention, is given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a vertically sectioned front view of a permanent magnet typethree-phase stepping motor in accordance with the present invention;

FIG. 1B is a vertically sectioned side view of the stepping motor shownin FIG 1A;

FIG. 2A is a perspective view of a cylindrical magnetic rotor of apermanent magnet type three-phase stepping motor in accordance with thepresent invention;

FIG. 2B is a perspective view of a hybrid type rotor of the steppingmotor shown in FIG. 1B;

FIG. 3 is an explanation view of a relation between stator teeth androtor teeth;

FIG. 4 is an explanation view of a relation between stator teeth androtor teeth;

FIG. 5 shows a transistor circuit of a star connection for drivingthree-phase windings of a permanent magnet type three-phase steppingmotor in accordance with the present invention;

FIG. 6 shows a transistor circuit of a delta connection for drivingthree-phase windings of a permanent magnet type three-phase steppingmotor in accordance with the present invention;

FIG. 7 shows a transistor circuit for independently driving three-phasewindings of a permanent magnet type three-phase stepping motor inaccordance with the present invention;

FIG. 8 is an explanation view of unbalanced radial forces generated bythe excitation of three-phase windings of a permanent magnet typethree-phase stepping motor in accordance with the present invention;

FIG. 9 is an explanation view of a cogging torque generated in athree-phase stepping motor; and

FIG. 10 is a graph explaining a cogging torque in a permanent magnettype three-phase stepping motor in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A first embodiment of the present invention will now be explained withreference to the drawings.

In FIG. 1A and FIG. 1B, a reference numeral 1 denotes a stator havingthree main poles, 2 denotes two magnetic rotor elements of hybrid type(HB), rotationally shifted from each other by a ½ tooth pitch inposition of their magnetic teeth, 3 denotes windings each wound aroundeach stator main pole for forming each phase, 4 denotes a rotor shaft, 5denotes a permanent magnet held by the two magnetic rotor elements 2 andmagnetized so as to form N and S poles in the axial direction thereof, 6and 7 denote brackets of non-magnetic material, such as aluminum havingportions extending in the axial direction for supporting the innerperipheral surface of the stator 1 so as to form an air gap between thestator 1 and the rotor 2, and 8 denotes bearings.

In the three-phase hybrid type stepping motor of the present invention,a pitch of stator pole teeth is set smaller than a pitch of the smallrotor teeth, and a number m of stator pole teeth of one main pole isspecified to (Nr±2^(k))/3, and Nr is specified to (3n±1), where m is aninteger, Nr is a number of small rotor teeth, n is an integer not lessthan 1, and k is an integer not less than 1 and not more than 4.

In the stepping motor of this structure, the torque can be increased inproportion to Nr or m substantially. In other words, the torque can beincreased by setting the number m to the largest value and the pitch ofthe stator pole teeth smaller than the pitch of the small rotor teeth,while suppressing the vibration, and preventing the reduction of thetorque.

FIG. 1A and FIG. 1B show a stepping motor realized under such conditionsthat n is 17, Nr is 50, k is 1, and m is 16. The present invention canbe applied not only to a three-phase motor having a HB type rotor, butalso a three-phase motor having a rotor wherein N poles and S poles areformed alternately on the outer peripheral surface of a cylindricalmagnet 9 so as to have a pole pair number of Nr, combined with a HB typestator. FIG. 2A shows an appearance of the cylindrical permanent magnetrotor for comparison with that of the HB type rotor shown in FIG. 2B. InFIG. 2A, reference numeral 4 denotes a rotor shaft and 9 denotes acylindrical permanent magnet.

A second embodiment of the present invention will be explained. Theharmonic wave in the air gap and the cogging torque due to the magneticflux of the permanent magnet can be reduced, if the pitch of the statorpole teeth shown in FIG. 1A is set smaller than that of the small rotorteeth. In order to obtain a good result, it is necessary to know whatkinds of harmonic wave are included in the cogging torque or torquewhich is generated when the windings of the permanent magnet typethree-phase stepping motor are excited by electric current. It is noted,however, that the cogging torque of the three-phase stepping motorrelates mainly to the sixth harmonic wave and the torque due to thecurrent relates to the odd number harmonic wave.

Formula 5 shows the magnetic flux due to the permanent magnet for eachphase in the three-phase stepping motor.

$\begin{matrix}{ {{3\; P_{0}}\operatorname{>>}{P_{m}.\begin{matrix}{\Phi_{A} = {( {P_{m}{F_{m}/3}P_{0}} )P_{A}}} \\{\Phi_{B} = {( {P_{m}{F_{m}/3}P_{0}} )P_{B}}}\end{matrix}}} \}{\Phi_{C} = {( {P_{m}{F_{m}/3}P_{0}} )P_{C}}}} & (5)\end{matrix}$Formula 6 shows permeances P_(A,) P_(B,) P_(C,) {overscore (P)}_(A,){overscore (P)}_(B) and {overscore (P)}_(C) of each phase in thethree-phase stepping motor.

$\begin{matrix} {{P_{A} = {P_{0}\{ {1 + {\sum{k_{n}\cos\;{n( {N_{r}\theta} )}}}} \}}}\begin{matrix}\begin{matrix}\begin{matrix}{P_{B} = {P_{0}\{ {1 + {\sum{k_{n}\cos\;{n( {{N_{r}\theta} + {2\;{\pi/3}}} )}}}} \}}} \\{P_{C} = {P_{0}\{ {1 + {\sum{k_{n}\cos\;{n( {{N_{r}\theta} + {4\;\pi\; 3}} )}}}} \}}}\end{matrix} \\{{\overset{\_}{P}}_{A} = {P_{0}\{ {1 + {\sum{k_{n}\cos\;{n( {{N_{r}\theta} + \;\pi} )}}}} \}}}\end{matrix} \\{{\overset{\_}{P}}_{B} = {P_{0}\{ {1 + {\sum{k_{n}\cos\;{n( {{N_{r}\theta} + {2\;{\pi/3}} + \pi} )}}}} \}}} \\{{\overset{\_}{P}}_{C} = {P_{0}\{ {1 + {\sum{k_{n}\cos\;{n( {{N_{r}\theta} + {4\;{\pi/3}} + \pi} )}}}} \}}}\end{matrix}} \} & (6)\end{matrix}$Here, Fm is a magnetomotive force of magnet, Pm is a permeance ofmagnet, and P_(o) is the constant of permeance of each phase. A coggingtorque T_(A) of phase A can be calculated from Formula 7.

$\begin{matrix}{T_{A} = {{- ( {1/2} )}\delta{\{ {\Phi_{A}^{2}/P_{A}} \}/\delta}\;\theta}} & (7) \\{\mspace{25mu}{= ( {{- N_{r}}P_{m}^{2}{F_{m}^{2}/18}P_{0}} )}} & \; \\{\mspace{56mu}( {{k_{1}\sin\; N_{r}\theta} + {2\; k_{2}\sin\; 2\; N_{r}\theta} + {3\; k_{3}\sin\; 3\; N_{r}\theta} + \ldots}\mspace{11mu} )} & \;\end{matrix}$Similarly thereto, cogging torques of phase B, phase C, phase Ā, phase{overscore (B)} and phase {overscore (C)} can be calculated. A coggingtorque T obtained by adding harmonic waves of from n=1 (fundamentalwave) to n=12 (twelfth harmonic wave) through the phase A to the phase{overscore (C)}, can be expressed by Formula 8.T=(−2N _(r) P _(m) ² F _(m) ² /P ₀)(k ₆ sin 6N _(r)θ+2k ₁₂ sin 12N_(r)θ+ . . . )  (8)

It should be understood that it is effective to cancel the sixthharmonic wave mainly, because the cogging torque T is composed of sixthand twelfth harmonic waves. Accordingly, it is preferable to determine adeviation angle α between the pitch of the stator pole teeth and thepitch of the small rotor teeth so as to eliminate the sixth content ofthe permeance forming the sixth harmonic wave in case of the coggingtorque.

On the other hand, a current torque T_(CA) of phase A is EI/ω m, where Eis an induced electromotive force, I is a current and ωm is a mechanicalangular velocity. The induced electromotive force E can be expressed byFormula 9 and T_(CA) can be expressed by Formula 10.E=dΦ/dt=dQ(P _(A) −{overscore (P)} _(A))/dt  (9)T _(CA)=(I/ωm)dQ(P _(A) −{overscore (P)} _(A))/dt  (10)(P_(A)−{overscore (P)}_(A)) can be obtained from Formula 6, and it isfound that the constant P_(o) and even number order harmonic waves areeliminated, but odd number order harmonic waves are remained. The thirdharmonic wave has the maximum width in the odd number order harmonicwaves. Accordingly, it is preferable to determine the deviation angle αbetween the pitch of the stator pole teeth and the pitch of the smallrotor teeth so as to eliminate the third order content of the permeanceforming the third harmonic wave in case of the current torque.

A manner for eliminating the third and sixth harmonic contents in theair gap will be explained hereunder.

FIG. 3 shows a relation between one main stator pole and a correspondingrotor portion in case that the number of the stator pole teeth is aneven number, that is, six. FIG. 4 shows a relation between one mainstator pole and the corresponding rotor portion in case that the numberof the stator pole teeth is an odd number, that is, five. In each case,the pitch of the stator pole teeth is deviated by α (electrical angle)from the pitch of the small rotor teeth. For example, the fundamentalwave P1 of permeance in FIG. 3 is expressed by Formula 11, whereas thethird permeance P3 is expressed by Formula 12.

$\begin{matrix}{P_{1} = {\frac{2}{6}{P_{0}( {{\cos\frac{\alpha}{2}} + {\cos\frac{3\alpha}{2}} + {\cos\frac{5\alpha}{2}}} )}}} & (11) \\{P_{3} = {\frac{2}{6}{P_{0}( {{\cos\; 3 \times \frac{\alpha}{2}} + {\cos\; 3 \times \frac{3\alpha}{2}} + {\cos\; 3 \times \frac{5\alpha}{2}}} )}}} & (12)\end{matrix}$

Accordingly, the third harmonic wave can be eliminated by setting P3 tozero in Formula 12 and obtaining the electrical angle α. Similarly, forder permeance in case that m is an even number can be eliminated byestablishing Formula 13.

$\begin{matrix}{{\sum\limits_{K = 1}^{m/2}{\cos\;{f( {K - \frac{1}{2}} )}\alpha}} = 0} & (13)\end{matrix}$

Accordingly, the electrical angle α is obtained by setting f to 3 inFormula 13 in order to eliminate the third permeance. Further, Formula14 is established in order to eliminate the seventh permeance.

$\begin{matrix}{{\sum\limits_{k = 1}^{m/2}\;{\cos\; 7( {k - \frac{1}{2}} )\alpha_{2}}} = 0} & (14)\end{matrix}$

As stated above, in order to eliminate the sixth harmonic wave, therange of α is specified as α₂≦α≦α₁, where α₁ is a value when f is 5 andα₂ is a value when f is 7. Similarly, in order to eliminate the thirdharmonic wave, the range of α can be specified as α₂≦α≦α₁, where α₁ is avalue when f is 2 and α₂ is a value when f is 4.

In order to eliminate f order permeance in case that m is an odd numberin the general expression, the electrical angle α in Formula 15 isobtained.

$\begin{matrix}{{1 + {\sum\limits_{k = 1}^{{({m - 1})}/2}\;{\cos\; f\; k\;\alpha}}} = 0} & (15)\end{matrix}$

In order to eliminate the seventh order permeance, Formula 16 isestablished.

$\begin{matrix}{{1 + {\sum\limits_{k = 1}^{{({m - 1})}/2}\;{\cos\; 7\; k\;\alpha_{2}}}} = 0} & (16)\end{matrix}$

Then, the electrical angles α₁ and α₂ are select as α₂≦α≦α₁.

By making the electrical angle α as stated above, the vibration of themotor having three main stator poles wherein the punched silicon steelplates cannot be piled while changing angular position by 90 degrees canbe reduced. It is not always necessary to select the electrical angle αstrictly in order to eliminate the third and sixth harmonic waves. It iseffective to reduce the third and sixth harmonic waves for thethree-phase motor. The amplitude of the harmonic wave becomes small ifthe number of the harmonic order is increased. Accordingly, it may besupposed easily that it is effective to eliminate a low order harmonicwave of large amplitude such as the third harmonic in order to reducethe vibration. In general, it is possible to select the electrical angleα by investigating the order of the harmonic wave of large amplitude inthe induced voltage of the motor. Normally, the optimum value of α isselected in the range of α to cancel the second to seventh harmonicwaves.

In order to carry out the three-terminal exciting for the three-phasestepping motor, six transistors Tr1 to Tr6 are connected in bridge andconnected to the three stator windings of a star connection as shown inFIG. 5 or of a delta connection as shown in FIG. 6. In case that thetwo-phase exciting is carried out in the stepping motor having threemain stator poles shown in FIG. 1A and FIG. 1B, only two main statorpoles are excited, so that a force in the radial direction applied tothe rotor is not cancelled, that a force in the radial direction isapplied always to the bearing or the like, and that the noise becomeslarge. The two-phase exciting can be carried out by switching ON thetransistors Tr1 and Tr4 or the transistors Tr5 and Tr4, for example atthe same time in the star connection shown in FIG. 5. On the contrarythereto, if the transistors Tr1, Tr4 and Tr6 are switched ON at the sametime, the three-phase exciting can be carried out, so that an unbalanceforce in the radial direction can be reduced to about 13%. In the deltaconnection shown in FIG. 6, the two-phase exciting can be carried out byswitching ON the transistors Tr1 and Tr4 at the same time. However, ifthe three-phase exciting is carried out, there are no idle windings, sothat a torque can be increased. Further, if the three-phase windings aredivided and connected to the transistors as shown in FIG. 7, therequired number of transistors becomes twice. However, the freedom ofthe exciting manner is increased remarkably, and the unbalance force inthe radial direction can be cancelled perfectly by controlling thecurrent in each phase independently even in the stepping motor havingthree main stator poles.

The reason why the unbalance force of about 13% is generated by thethree-terminal exciting of the three-phase stepping motor having threemain stator poles will be explained with reference to FIG. 8. In casethat three windings of star or delta connection are excited, if acurrent I is passed through one main pole winding of first phase, acurrent of I/2 is passed through each of the windings of second andthird phases. When the stator pole teeth of the first phase are facedperfectly to the corresponding small rotor teeth, the stator pole teethof the second phase or the third phase are deviated by 30° (electricalangle) from the corresponding small rotor teeth. Accordingly, if anunbalance force in the radial direction F1 is I in FIG. 8, thenF2=F3=(I/2)cos 30° and F2+F3=0.87 I. That is, the unbalance forcedirection of about 13% of F1 is generated. However, in the system shownin FIG. 7, the unbalance force in the radial direction can be cancelled,if a current passing through a winding wound around the stator magneticpole, of which stator pole teeth are faced perfectly to thecorresponding small rotor teeth is reduced by about 13% from the normalvalue. On the contrary thereto, it is considered that a current passingthrough the other winding may be increased.

Other than the three-phase motor having three main stator poles, athree-phase motor having a multiple number of three, such as six, nineor twelve main stator poles is considered as a practical motor. In sucha case, the other manner for reducing the vibration and minimizing thecogging torque which is generated by only the permanent magnet of therotor when the windings are not excited will be explained. The coggingtorque is generated due to the harmonic content of the field, and theharmonic content is affected largely by the tooth width of the statorand the tooth width of the rotor. It is recognized experimentally andobtained by the magnetic field analysis that an element for varying thecogging torque is varied by the sum of the tooth width of the stator andthe tooth width of the rotor. FIG. 9 shows a relation between a statorpole tooth of a width Ws and a small rotor tooth of a width Wr facingeach other. In FIG. 9, Dr is an outer diameter of the rotor and Ds is aninner diameter of the stator. A diameter of the center of an air gapbetween the rotor and the stator is (Ds+Dr)/2. FIG. 10 shows a relationbetween a ratio h of (Nr/π)(Ws+Wr)/(Ds+Dr) and the cogging torque Tc,wherein the cogging torque Tc becomes minimum value 1 when h is 0.4.Further, it is understood from FIG. 10 that the cogging torque isreduced largely if the ratio h is selected between 0.3 and 0.5 or to±25% of 0.4.

It is an effective manner in the permanent magnet type three-phasestepping motor having 3 L or 3K pieces of main stator pole to select theh as 0.3≦h≦0.5.

According to the present invention, a permanent magnet type three-phasestepping motor comprises a stator and a rotor arranged concentricallywith the stator and with an air gap therebetween, said stator having 3 Lpieces of stator pole, and stator windings of three-phase each woundaround each stator pole, each of said stator poles having a plurality ofstator pole teeth, said rotor having two split rotor elements and apermanent magnet held between the rotor elements and magnetized so as toform N and S poles in the axial direction thereof, and a plurality (Nr)of small rotor teeth formed at a regular pitch on the outer peripheralsurface of each of said rotor elements, said two split rotor elementsbeing circumferentially shifted from each other by a ½ pitch of thesmall rotor teeth, or said rotor being a cylindrical permanent magnetmagnetized so as to form a plurality (Nr) of N and S pole pairsalternately at a regular pitch in the circumferential direction thereof,wherein a condition of 0.3≦{(Nr/π)(Ws+Wr)/(Ds+Dr)}≦0.5 is established,where L is an integer not less than 1, Nr is L (3n ±1 ), n is an integernot less than 1, Ds is an inner diameter of the stator, Dr is an outerdiameter of the rotor, Ws is a width of the stator pole tooth, and Wr isa width of the rotor tooth or rotor pole.

Further, according to the present invention, a permanent magnet typethree-phase stepping motor comprises a stator and a rotor arrangedconcentrically with the stator and with an air gap therebetween, saidstator having 3K pieces of stator pole, and stator windings ofthree-phase each wound around each stator pole, each of said statorpoles having a plurality of stator pole teeth, said rotor having twosplit rotor elements and a permanent magnet held between the rotorelements and magnetized so as to form N and S poles in the axialdirection thereof, and a plurality (Nr) of small rotor teeth formed at aregular pitch on the outer peripheral surface of each of said rotorelements, said two split rotor elements being circumferentially shiftedfrom each other by a ½ pitch of the small rotor teeth, or said rotorbeing a cylindrical permanent magnet magnetized so as to form aplurality (Nr) of N and S pole pairs at a regular pitch in thecircumferential direction thereof, wherein a condition of0.3≦{(Nr/π)(Ws+Wr)/(Ds+Dr)}≦0.5 is established, where K is an evennumber not less than 2, Nr is K/2(6n±1), n is an integer not less than1, Ds is an inner diameter of the stator, Dr is an outer diameter of therotor, Ws is a width of the stator pole tooth, and Wr is a width of therotor tooth or rotor pole.

The above manner is suitable for a motor wherein L is 2, n is 8, thenumber of main stator poles is 6, Nr is 50 and the step angle is 1.2°,wherein L is 4, n is 8, the number of main stator poles is 12, Nr is 100and the step angle is 0.6°, or wherein K is 4, n is 4, the number ofmain stator poles is 12, Nr is 50 and the step angle is 1.2°. The numberNr of the small rotor teeth can be obtained from Formula 17 and Formula18, where n is a natural number.60°/N _(r)=−/+{360°/3L−360°(n±½)/N _(r)}  (17)60°/N _(r)=−/+{360°/3K−360°n/N _(r)}  (18)

Each of the left and right sides of Formula 17 shows a step angle(mechanical angle) of the three-phase motor and accordingly an equationof Nr=L(3n±1) can be obtained therefrom.

Similarly, an equation of Nr=K/2(6n±1) can be obtained from Formula 18.

According to the present invention, a permanent magnet type three-phasestepping motor simple in construction, small in vibration, small inunbalance force in the radial direction, and high in torque can bemanufactured with low cost.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

1. A permanent magnet type three-phase stepping motor comprising astator and a rotor said rotor arranged concentrically with the statorwith an air gap therebetween, said stator having three stator poles andstator windings of three-phase each wound around each stator pole, eachof said stator poles having m pieces of stator pole teeth, said rotorhaving two split rotor elements and a permanent magnet held between thetwo split rotor elements and magnetized to form N and S poles in theaxial direction thereof, and a plurality (Nr) of small rotor teethformed at a regular pitch on an outer peripheral surface of each of saidtwo split rotor elements, said two split rotor elements beingcircumferentially shifted from each other by a ½ pitch of the smallrotor teeth; wherein a pitch of the stator pole teeth is smaller thanthe pitch of the small rotor teeth, m is an integer equal to(Nr±2^(k))/3, Nr is 3n±1, wherein n is an integer not less than 1, and kis an integer not less than 1 and not more than
 4. 2. The permanentmagnet type three-phase stepping motor as claimed in claim 1, wherein athree-phase exciting is carried out with respect to the three-phasewindings and a current passing through each of the three-phase windingsis controlled so as to eliminate an unbalance force in a radialdirection.
 3. The permanent magnet type three-phase stepping motor asclaimed in claim 1, wherein the rotor is held by a bracket ofnon-magnetic material, the bracket having a portion extending inwardlyin the axial direction for supporting the inner peripheral surface ofthe stator poles.